Method for performing random access procedure in wireless communication system

ABSTRACT

There is provided a method for enabling a user equipment to perform a random access procedure in a wireless communication system. The method includes transmitting a random access preamble and receiving a random access response as a response to the random access preamble. The random access response is addressed by a random access identifier. The random access identifier is mapped to radio resources used for transmitting the random access preamble. The user equipment can immediately confirm its random access response and perform further efficient random access procedure.

This application claims the benefit of PCT/KR2007/004016 filed on Aug.22, 2007, U.S. Provisional application No. 60,823,371 filed on Aug. 23,2006, U.S. Provisional application No. 60/896,250 filed on Mar. 21,2007, and Korean Patent Application No. 10-2007-0082053 filed on Aug.15, 2007, the contents of which are hereby incorporated herein byreference for all purposes in their entirety.

TECHNICAL FIELD

The present invention relates to wireless communication. Morespecifically, the invention relates to a method of performing a randomaccess procedure in a wireless communication system.

BACKGROUND ART

Third generation partnership project (3GPP) mobile communication systemsbased on a wideband code division multiple access (WCDMA) radio accesstechnique are widely deployed all over the world. High speed downlinkpacket access (HSDPA) that can be defined as a first evolutionary stageof WCDMA provides 3GPP with highly competitive radio access technique inthe mid-term future. However, since requirements and expectations ofusers and service providers are continuously increased and developmentsof competing radio access techniques are continuously in progress, newtechnical evolutions in 3GPP are required to secure competitiveness inthe future. Decrease of cost per bit, increase in service availability,flexible use of frequency bands, simple architecture and open interface,low power consumption of a user equipment and the like are suggested asrequirements of next generation communication systems.

Generally, one or more cells are allocated to a base station. Aplurality of user equipments can be placed in a cell. A user equipmentgenerally performs random access procedure before accessing to anetwork. The objectives of the random access procedure may include 1)initial access, 2) handover, 3) scheduling request (request for radioresources), 4) timing synchronization, and the like.

Random access procedure in the initial access is performed as follows. Auser equipment is turned on and tries to initially access a network. Theuser equipment establishes downlink synchronization with the network andreceives system information from the network. Since the user equipmentcurrently does not establish timing synchronization and does not haveany uplink padio resources, the user equipment performs the randomaccess procedure. The user equipment requests uplink radio resources fortransmitting an access request message through the random accessprocedure. The network which receives the request for the uplink radioresources allocates the uplink radio resources to the user equipment.The user equipment can transmit the access request message to thenetwork through the uplink radio resources.

The random access procedure can be classified into a contention basedrandom access procedure and a non-contention based random accessprocedure. The most outstanding difference between the contention basedrandom access procedure and the non-contention based random accessprocedure is whether a signature used for a random access preamble isdedicated to a user equipment. In the non-contention based random accessprocedure, since a user equipment uses a dedicated signature assignedonly to itself as the random access preamble, a collision with otheruser equipments do not occur. The collision means that two or more userequipments simultaneously try a random access procedure using the samerandom access preamble through the same radio resources. In thecontention based random access procedure, since a user equipmentarbitrarily selects one signature among shared signatures as the randomaccess preamble, the collision may occur.

Random access procedure in a wireless communication system based onconventional WCDMA system is described below.

The random access procedure in WCDMA system uses a physical randomaccess channel (PRACH) as an uplink physical channel and an acquisitionindicator channel (AICH) as a downlink physical channel. For thestructures of the PRACH and the AICH, sections 5.2.2.1 and 5.3.3.7 of3GPP TS 25.211 V7.0.0 (2006-03) “Technical Specification Group RadioAccess Network; Physical channels and mapping of transport channels ontophysical channels (FDD) (Release 7)” can be referenced.

The PRACH can be divided into a preamble part and a message part. Thepreamble part carries a signature. At first, a physical layer of a userequipment selects an access slot and the signature. The signature istransmitted through the access slot as a random access preamble. As anexample of signatures, section 4.3.3.3 of 3GPP TS 25.213 V7.0.0 (2006March) “Technical Specification Group Radio Access Network; Spreadingand modulation (FDD) (Release 7)” can be referenced. The signature isselected from sixteen signatures. The random access preamble can betransmitted for 1.33 ms duration of the access slot.

If the user equipment transmits the random access preamble to a network,the network transmits a response message to the user equipment throughthe AICH. The response message includes the signature of the randomaccess preamble so that the user equipment can identify the responsemessage as the user equipment's. The network transmits anacknowledgement (ACK) or a negative-acknowledgement (NACK) to the userequipment through the response message. If the user equipment receivesthe ACK from the network, the user equipment transmits the message partusing an orthogonal variable spreading factor (OVSF) code correspondingto the signature. If the user equipment receives the NACK from thenetwork, the user equipment transmits another random access preamblethrough the PRACH after a certain time period. If the user equipmentdoes not receive the response message corresponding to the random accesspreamble, the user equipment transmits a new random access preamble withpower whose level is higher by one step than that of the previous randomaccess preamble.

The network transmits the response message as a response to the randomaccess preamble from the user equipment. The response message includesthe signature that is the same as the signature used for the randomaccess preamble. The user equipment identifies that the response messageis the response to its random access preamble through the signature ofthe response message.

If the response message in the random access procedure carries a varietyof information as much as possible, the efficiency of the random accessprocedure can be improved. For example, if the response message isgenerated by an upper layer of the physical layer, a variety ofinformation can be transmitted at a time through the response message.

Since the random access procedure is used for a variety of purposes, itis required that the random access procedure should be efficientlyconfigured to enhance performance of the wireless communication systemas a whole. Therefore, there is a need for a method that can moreefficiently process the random access procedure.

DISCLOSURE OF INVENTION Technical Problem

The object of the present invention is to perform a random accessprocedure using a random access identifier.

Technical Solution

In one aspect, there is provided with a method for enabling a userequipment to perform a random access procedure in a wirelesscommunication system. The method includes transmitting a random accesspreamble and receiving a random access response as a response to therandom access preamble. The random access response is addressed by arandom access identifier. The random access identifier is mapped toradio resources used for transmitting the random access preamble and isan identifier for identifying the user equipment performing the randomaccess procedure.

In another aspect, there is provided with a method for enabling a userequipment to perform a random access procedure in a wirelesscommunication system. The method includes receiving mapping informationwhich is information on mapping between radio resources used fortransmitting a random access preamble and a random access identifier foridentifying the user equipment performing the random access procedure,transmitting the random access preamble and receiving the random accessidentifier mapped to the radio resources used for transmitting therandom access preamble.

In still another aspect, there is provided with a method for enabling abase station to perform a random access procedure in a wirelesscommunication system. The method includes transmitting mappinginformation which is information on mapping between radio resources usedfor transmitting a random access preamble and a random access identifierfor identifying a user equipment performing the random access procedure,receiving the random access preamble, obtaining the random accessidentifier using the mapping information and the radio resources usedfor transmitting the random access preamble and transmitting a randomaccess response addressed by the random access identifier.

Advantageous Effects

A random access identifier is mapped to radio resources used in theprocess of performing a random access procedure between a user equipmentand a network. Through the mapping of the random access identifier tothe radio resources, a user equipment can immediately confirm its randomaccess response and efficiently perform the random access procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a wireless communication system.

FIG. 2 is a block diagram showing a functional split between evolveduniversal terrestrial radio access network (E-UTRAN) and evolved packetcore (EPC).

FIG. 3 is a block diagram showing the radio protocol architecture of auser plane.

FIG. 4 is a block diagram showing the radio protocol architecture of acontrol plane.

FIG. 5 is a flowchart illustrating a random access procedure accordingto an embodiment of the present invention.

FIG. 6 is a view showing an example of a random access procedure.

FIG. 7 is a flowchart illustrating a random access procedure accordingto another embodiment of the present invention.

MODE FOR THE INVENTION

FIG. 1 is a block diagram showing a wireless communication system. Thismay be the network architecture of an evolved-universal mobiletelecommunications system (E-UMTS). The E-UMTS may be referred to as along term evolution (LTE) system. The wireless communication system iswidely deployed to provide a variety of communication services such asaudio data, packet data, or the like.

Referring to FIG. 1, evolved-UMTS terrestrial radio access network(E-UTRAN) includes a base station (BS) 20 for providing control planeand user plane.

A user equipment (UE) 10 can be fixed or mobile and can be referred toas other terminologies such as a mobile station (MS), user terminal(UT), subscriber station (SS), wireless device, or the like. The basestation 20 generally means a fixed station that communicates with theuser equipment 10 and can be referred to as other terminologies such asan evolved-NodeB (eNB), base transceiver system (BTS), access point, orthe like. The base station 20 cane serves for at least one cell. Aninterface for transmitting user traffics or control traffics can be usedbetween base stations 20. Hereinafter, downlink means communication fromthe base station 20 to the user equipment 10, and uplink meanscommunication from the user equipment 10 to the base station 20.

The base stations 20 can be interconnected with each other by means ofan X2 interface. The base station 20 is connected to evolved packet core(EPC), more specifically to a mobility management entity/systemarchitecture evolution (SAE/MME) gateway 30, through an S1 interface.The S1 interface supports a many-to-many relation between the basestation 20 and the MME/SAE gateway 30.

FIG. 2 is a block diagram showing a functional split between E-UTRAN andEPC.

Referring to FIG. 2, slant lined blocks represent radio protocol layers,and other blocks show functional entities of the control plane.

The base station hosts following functions: (1) functions for radioresource management such as a radio bearer control, radio admissioncontrol, connection mobility control and dynamic allocation of resourcesto user equipments in both uplink and downlink (scheduling), (2)Internet protocol (IP) header compression and encryption of user datastreams, (3) routing of user plane data towards an SAE gateway, (4)scheduling and transmission of paging messages, (5) scheduling andtransmission of broadcast information and (6) measurement andmeasurement reporting configuration for mobility and scheduling.

The MME hosts following functions: (1) distribution of paging messagesto the base stations, (2) security control, (3) idle state mobilitycontrol, (4) SAE bearer control and (5) ciphering and integrityprotection of non-access stratum (NAS) signaling.

The SAE gateway hosts following functions: (1) termination of user planepackets for paging reasons and (2) switching of user plane for supportof user equipment mobility.

Layers of the radio interface protocol between the user equipment andthe base station can be classified into L1 (a first layer), L2 (a secondlayer), and L3 (a third layer) based on the lower three layers of theopen system interconnection (OSI) model that is well-known tocommunication systems. The physical layer belonging to the first layerprovides an information transfer service using a physical channel. Aradio resource control (RCC) layer belonging to the third layer servesto control radio resources between the user equipment and the network.The UE and the network exchange RRC messages via the RRC layer.

FIG. 3 is a block diagram showing the radio protocol architecture of auser plane, and FIG. 4 is a block diagram showing the radio protocolarchitecture of a control plane. These show the architectures of theradio interface protocol between the user equipment and the E-UTRAN. Thedata plane is a protocol stack for transmitting traffic data, and thecontrol plane is a protocol stack for transmitting control signals.

Referring to FIGS. 3 and 4, a physical layer provides an informationtransfer service to an upper layer using a physical channel. Thephysical layer is connected to the medium access control (MAC) layerthrough a transport channel, and data are transferred between the MAClayer and the physical layer via a transport channel. Data moves betweenthe MAC layer and the physical layer via the transport channel. Datamoves between different physical layers, that is, a physical layer for atransmitter and a physical layer for a receiver, via the physicalchannel. The physical layer can modulate data in the orthogonalfrequency division multiplexing (OFDM) scheme and may use time andfrequency (subcarrier) as radio resources.

The MAC layer which belongs to the second layer provides a service to aradio link control (RLC) layer, which is the upper layer of the MAClayer, via a logical channel. The RLC layer which belongs to the secondlayer supports reliable data transfer.

A packet data convergence protocol (PDCP) layer which belongs to thesecond layer performs header compression function. When transmitting anInternet Protocol (IP) packet such as an IPv4 packet or an IPv6 packet,the header of the IP packet may contain relatively large and unnecessarycontrol information. The PDCP layer reduces the header size of the IPpacket so as to efficiently transmit the IP packet.

The radio resource control (RRC) layer which belongs to the third layeris defined only on the control plane. The RRC layer serves to controlthe logical channel, the transport channel and the physical channel inassociation with configuration, reconfiguration and release of a radiobearer (RB). The RB is a service provided by the second layer for datatransmission between the user equipment and the E-UTRAN. If an RCCconnection is established between the RCC of the user equipment and theRCC of the network, the user equipment is in an RCC connection mode, andotherwise, the user equipment is in an RCC idle mode.

A NAS layer which belongs to the upper layer of the RCC layer performsauthentication, SAE bearer management, security control, and the like.

A downlink transport channel for transmitting data from the network tothe user equipment includes a broadcast channel (BCH) for transmittingsystem information and a downlink-shared channel (DL-SCH) fortransmitting user traffics or control messages. Multicast traffics orbroadcast traffics can be transmitted via the DL-SCH or adownlink-multicast channel (DL-MCH). A downlink physical channel mappedto the downlink transport channel includes a physical downlink sharedchannel (PDSCH) mapped to the DL-SCH and a physical downlink controlchannel (PDCCH) for transmitting control signals for the PDSCH.

An uplink transport channel for transmitting data from the userequipment to the network includes a random access channel (RACH) fortransmitting initial access messages and an uplink-shared channel(UL-SCH) for transmitting user traffics or control messages. An uplinkphysical channel includes a physical random access channel (PRACH)mapped to the RACH and a physical uplink shared channel (PUSCH) mappedto the UL-SCH.

The RACH which is the uplink transport channel is used to transmitshort-length data through the uplink. Some of RCC messages, such as anRRC connection request message, cell update message, URA update message,and the like, are transmitted via the RACH. The logical channel, such ascommon control channel (CCCH), dedicated control channel (DCCH) ordedicated traffic channel (DTCH), can be mapped to the RACH. The RACH ismapped to the physical channel, PRACH.

FIG. 5 is a flowchart illustrating a random access procedure accordingto an embodiment of the present invention.

Referring to FIG. 5, a user equipment receives mapping information froma base station S210. The mapping information is the information onmapping relation between radio resources used for transmitting a randomaccess preamble and a random access identifier (RA-ID).

The RA-ID is an identifier to identify the user equipment performing arandom access procedure. The RA-ID identifies a random access occasionor a random access group. The random access group is a set of randomaccess occasions. The random access occasion means time-frequencyresources used for transmitting the random access preamble. When theRA-ID is arrived, the user equipment reads information addressed by theRA-ID. On the contrary, a cell-radio network temporary identifier(C-RNTI) is an identifier to identify the user equipment within a cell.

The mapping information includes the information on mapping relationbetween radio resources used for transmitting the random access preambleand the RA-ID. The mapping information may be an RRC message. Themapping information can be transmitted through system information or apaging message. The radio resources are time-frequency resources usedfor transmitting the random access preamble. The radio resources mayinclude time resources and/or frequency resources. The radio resourcesmay be represented as transmission intervals, resource blocks, and thelike that are used for transmitting the random access preamble. If therandom access preamble is transmitted via the PRACH, the radio resourcescan be time-frequency resources allocated for the PRACH.

The user equipment selects a signature and radio resources fortransmitting the random access preamble S220. As the radio resources areselected, the user equipment selects an RA-ID corresponding to the radioresources based on the mapping information.

The user equipment transmits the random access preamble to the basestation through the radio resources S230. The random access preamble isa physical layer message constructed in the physical layer. The randomaccess preamble is transmitted via the PRACH.

The base station transmits a random access response as a response to therandom access preamble S240. The random access response can be a MACmessage constructed in the MAC layer that is an upper layer of thephysical layer. The random access response can be transmitted via theDL-SCH. The random access response is addressed by the RA-ID transmittedvia an L1/L2 control channel. The L1/L2 control channel is a controlchannel associated with the DL-SCH. Through the L1/L2 control channel,the base station transmits the RA-ID corresponding to the radioresources through which the random access preamble is transmitted, usingthe mapping information. The user equipment that has transmitted therandom access preamble prepares to receive the random access responsebased on the RA-ID. If the RA-ID is received from the base station, theuser equipment receives a random access response addressed by the RA-ID.

The random access response can include at least any one of timingalignment information, initial uplink grant and a temporary C-RNTI. Thetiming alignment information is timing compensation information foruplink transmission. The initial uplink grant is ACK/NACK informationfor granting uplink transmission. The temporary C-RNTI is a C-RNTI thatis temporarily used before contention resolution is completed.

The user equipment performs scheduled uplink transmission via the UL-SCHS250.

The RA-ID used in the random access procedure is mapped to the radioresources used for transmitting the random access preamble. The RA-ID isan identifier used for addressing the random access response between thebase station and the user equipment when the random response istransmitted and received.

It is assumed that a random access interval is 10 ms. Radio resourcescomprising time and frequencies for transmitting at least one or morerandom access preambles is allocated for the random access interval. Theradio resources for performing random accesses are repeatedly allocatedfor every random access intervals. Since two or more random accesses maybe received for the random access interval, the base station maytransmit two or more random access responses. The user equipment has toidentify its own random access response. For example, it is assumed thatthe starting point of the Nth random access interval is T and the endingpoint of the Nth random access interval is T+10. It is assumed that afirst user equipment transmits a first random access preamble at T+2 anda second user equipment transmits a second random access preamble atT+3. Random access responses for the two random access preambles aretransmitted at T+7. At this point, there is ambiguity about which of thetwo random access responses is actually received by the first userequipment or the second user equipment.

If the RA-ID is in a mapping relation with the radio resources of therandom access preamble, the ambiguity can be resolved. If a first RA-IDis mapped to the first random access preamble and a second RA-ID ismapped to the second random access preamble, the first user equipmentcan immediately confirm a random access response addressed by the firstRA-ID as its response.

FIG. 6 is a view showing an example of a random access procedure.

Referring to FIG. 6, N user equipments (UE 1, UE 2, . . . , and UE N)respectively perform a random access procedure. Each of the userequipments transmits a random access preamble through selected radioresources. Each of the user equipments can transmit a random accesspreamble through radio resources different from those of others in atime and/or frequency domain.

A base station searches for RA-IDs corresponding to respective userequipments from the radio resources used by the received random accesspreambles using the mapping information. The base station transmits arandom access response addressed by the RA-ID for each of the userequipments. Each of the user equipments can identify its random accessresponse by receiving a random access response addressed by its RA-ID.

For example, the random access preamble of a first user equipment UE 1corresponds to the first RA-ID RA-ID 1, and the random access preambleof a second user equipment UE 2 corresponds to the second RA-ID RA-ID 2.The first user equipment UE 1 receives a random access responseaddressed by the first RA-ID RA-ID 1. Since the first user equipment UE1 knows that the RA-ID for the random access preamble transmitted byitself is the first RA-ID RA-ID 1 through the mapping information, itcan immediately receive the random access response. Accordingly, a upperlayer message for identifying a random access response is not needed.

A user equipment obtains mapping information between the radio resourcesused in the random access procedure and the RA-ID through systeminformation or the like. The user equipment selects a signature andradio resources and transmits a random access preamble through theselected signature and radio resources. Subsequently, the user equipmentsearches for the RA-ID through the L1/L2 control channel. The userequipment receives a DL-SCH message, i.e., its random access response,addressed by the RA-ID.

A user equipment maps the radio resources used in the process ofperforming a random access to a RA-ID. A random access response isaddressed by the RA-ID. Since it is not necessary to transmit upperlayer message for identifying the random access response, the number ofmessages exchanged between the user equipment and the base station canbe reduced, and power consumption of the user equipment can bedecreased.

FIG. 7 is a flowchart illustrating a random access procedure accordingto another embodiment of the present invention.

Referring to FIG. 7, a user equipment transmits a random access preambleto a base station through the selected radio resources S310. Here,unlike the embodiment of FIG. 5, the mapping information is nottransmitted through the system information, but both the user equipmentand the base station previously know the mapping information. The userequipment selects a signature and radio resources for transmitting therandom access preamble and searches for a RA-ID corresponding to theselected radio re sources based on previously stored mappinginformation.

The base station transmits a random access response as a response to therandom access preamble S320. The random access response is addressed bythe RA-ID transmitted via a L1/L2 control channel. The user equipmentthat has transmitted the random access preamble prepares to receive arandom access response based on the selected RA-ID. If the RA-ID isarrived from the base station, the user equipment receives the randomaccess response addressed by the RA-ID.

The user equipment performs scheduled uplink transmission via a UL-SCHS330.

The steps of a method described in connection with the embodimentsdisclosed herein may be implemented by hardware, software or acombination thereof. The hardware may be implemented by an applicationspecific integrated circuit (ASIC) that is designed to perform the abovefunction, a digital signal processing (DSP), a programmable logic device(PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, the other electronic unit, or acombination thereof. A module for performing the above function mayimplement the software. The software may be stored in a memory unit andexecuted by a processor. The memory unit or the processor may employ avariety of means that is well known to those skilled in the art.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims. Therefore, allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are intended to beembraced by the appended claims.

The invention claimed is:
 1. A method for enabling a user equipment toperform a random access procedure in a wireless communication system,the method comprising: selecting randomly a random access preamble froma set of random access preambles; selecting a random access occasionfrom a set of random access occasions, each random access occasion ofsaid set of random access occasions indicating possible time-frequencyresources used for transmitting said selected random access preamble;transmitting the selected random access preamble on the selected randomaccess occasion; determining a random access identifier to identify adownlink control channel for a random access response as a response tothe transmitted random access preamble, wherein the random accessidentifier is determined by the user equipment based on the selectedrandom access occasion; determining whether the downlink control channelidentified by the random access identifier is received; and when thedownlink control channel identified by the random access identifier isreceived, receiving the random access response via a downlink sharedchannel, the downlink shared channel being associated with theidentified downlink control channel.
 2. The method according to claim 1,further comprising receiving mapping information which is information onmapping between said each random access occasion and said random accessidentifier.
 3. The method according to claim 1, wherein the selectedrandom access preamble is constructed in a physical layer of the userequipment, and the random access response is constructed in an upperlayer of a physical layer of a base station.
 4. The method according toclaim 1, wherein the random access response comprises timing alignmentinformation, an initial uplink grant and a temporary Cell-Radio NetworkTemporary Identifier (C-RNTI).
 5. A user equipment configured forperforming a random access procedure in a wireless communication system,the user equipment comprising: a processor; and a memory coupled to theprocessor, the memory comprising instructions which, when executed bythe processor, cause the user equipment to: select randomly a randomaccess preamble from a set of random access preambles; select a randomaccess occasion from a set of random access occasions, each randomaccess occasion of said set of random access occasions indicatingpossible time-frequency resources used for transmitting said selectedrandom access preamble; transmit the selected random access preamble onthe selected random access occasion; determine a random accessidentifier to identify a downlink control channel for a random accessresponse as a response to the transmitted random access preamble,wherein the random access identifier is determined by the user equipmentbased on the selected random access occasion; determine whether thedownlink control channel identified by the random access identifier isreceived; and when the downlink control channel identified by the randomaccess identifier is received, receive the random access response via adownlink shared channel, the downlink shared channel being associatedwith the identified downlink control channel.
 6. The user equipmentaccording to claim 5, wherein the random access response comprisestiming alignment information, an initial uplink grant and a temporaryCell-Radio Network Temporary Identifier (C-RNTI).